fix: improve runtime ternary support by refining side-effect detection

packages/typegpu/src/core/buffer/bufferUsage.ts

@@ -140,7 +140,7 @@ class TgpuFixedBufferImpl<TData extends BaseData, TUsage extends BindableBufferU
       {
         [$internal]: true,
         get [$ownSnippet]() {
-          return snip(this, dataType, usage);
+          return snip(this, dataType, usage, /* possibleSideEffects */ false);
         },
         [$resolve]: (ctx) => ctx.resolve(this),
         toString: () => `${this.usage}:${getName(this) ?? '<unnamed>'}.$`,
@@ -262,7 +262,7 @@ export class TgpuLaidOutBufferImpl<TData extends BaseData, TUsage extends Bindab
       {
         [$internal]: true,
         get [$ownSnippet]() {
-          return snip(this, schema, usage);
+          return snip(this, schema, usage, /* possibleSideEffects */ false);
         },
         [$resolve]: (ctx) => ctx.resolve(this),
         toString: () => `${this.usage}:${getName(this) ?? '<unnamed>'}.$`,

packages/typegpu/src/core/constant/tgpuConstant.ts

@@ -128,7 +128,7 @@ class TgpuConstImpl<TDataType extends BaseData> implements TgpuConst<TDataType>,
       {
         [$internal]: true,
         get [$ownSnippet]() {
-          return snip(this, dataType, 'constant-immutable-def');
+          return snip(this, dataType, 'constant-immutable-def', /* possibleSideEffects */ false);
         },
         [$resolve]: (ctx) => ctx.resolve(this),
         toString: () => `const:${getName(this) ?? '<unnamed>'}.$`,

packages/typegpu/src/core/function/dualImpl.ts

@@ -1,4 +1,4 @@
-import { type MapValueToSnippet, snip } from '../../data/snippet.ts';
+import { type MapValueToSnippet, noSideEffects, snip } from '../../data/snippet.ts';
 import { setName } from '../../shared/meta.ts';
 import { $gpuCallable } from '../../shared/symbols.ts';
 import { tryConvertSnippet } from '../../tgsl/conversion.ts';
@@ -101,12 +101,17 @@ export function dualImpl<T extends AnyFn>(options: DualImplOptions<T>): DualFn<T
         }
       }
 
-      return snip(
+      const result = snip(
         options.codegenImpl(ctx, converted),
         concretize(returnType),
         // Functions give up ownership of their return value
         /* origin */ 'runtime',
       );
+
+      if (!args.some((a) => a.possibleSideEffects)) {
+        return noSideEffects(result);
+      }
+      return result;
     },
   };
 

packages/typegpu/src/resolutionCtx.ts

@@ -345,7 +345,7 @@ function createArgument(
     name,
     access: () => {
       used = true;
-      return snip(name, type, origin);
+      return snip(name, type, origin, /* possibleSideEffects */ false);
     },
     decoratedType: type,
     get used() {

packages/typegpu/src/tgsl/wgslGenerator.ts

@@ -447,6 +447,9 @@ ${this.ctx.pre}}`;
         type,
         // Result of an operation, so not a reference to anything
         /* origin */ 'runtime',
+        exprType === NODE.assignmentExpr ||
+          lhsExpr.possibleSideEffects ||
+          rhsExpr.possibleSideEffects,
       );
     }
 

packages/typegpu/tests/tgsl/ternaryOperator.test.ts

@@ -181,18 +181,18 @@ describe('ternary operator', () => {
     `);
   });
 
-  it('should throw when test is not comptime known', () => {
+  it('should generate select() for runtime condition with function params', () => {
     const myFn = tgpu.fn(
       [d.u32],
       d.u32,
     )((n) => {
       return n > 0 ? n : -n;
     });
 
-    expect(() => tgpu.resolve([myFn])).toThrowErrorMatchingInlineSnapshot(`
-      [Error: Resolution of the following tree failed:
-      - <root>
-      - fn:myFn: Ternary operator '(n > 0) ? n : (-n)' is invalid. For more complex branching, please use 'std.select' or if/else statements.]
+    expect(tgpu.resolve([myFn])).toMatchInlineSnapshot(`
+      "fn myFn(n: u32) -> u32 {
+        return select(-(n), n, (n > 0u));
+      }"
     `);
   });
 });

packages/typegpu/tests/tgsl/ternaryRuntime.test.ts

@@ -0,0 +1,165 @@
+import { describe, expect } from 'vitest';
+import { it } from 'typegpu-testing-utility';
+import tgpu, { d } from '../../src/index.js';
+
+describe('runtime ternary operator', () => {
+  it('should handle subtraction in branches with function params', () => {
+    const myFn = tgpu.fn(
+      [d.u32, d.u32],
+      d.u32,
+    )((b, w) => {
+      return b > w ? b - w : w - b;
+    });
+
+    expect(tgpu.resolve([myFn])).toMatchInlineSnapshot(`
+      "fn myFn(b: u32, w: u32) -> u32 {
+        return select((w - b), (b - w), (b > w));
+      }"
+    `);
+  });
+
+  it('should handle const array indexing in branches', () => {
+    const RotLut2Gpu = tgpu.const(d.arrayOf(d.u32, 2), [10, 20]);
+    const RotLut3Gpu = tgpu.const(d.arrayOf(d.u32, 3), [30, 40, 50]);
+
+    const myFn = tgpu.fn(
+      [d.u32, d.u32],
+      d.u32,
+    )((r, bitU) => {
+      return r === d.u32(2) ? (RotLut2Gpu.$[bitU] as number) : (RotLut3Gpu.$[bitU] as number);
+    });
+
+    expect(tgpu.resolve([myFn])).toMatchInlineSnapshot(`
+      "const RotLut2Gpu: array<u32, 2> = array<u32, 2>(10u, 20u);
+
+      const RotLut3Gpu: array<u32, 3> = array<u32, 3>(30u, 40u, 50u);
+
+      fn myFn(r: u32, bitU: u32) -> u32 {
+        return select(RotLut3Gpu[bitU], RotLut2Gpu[bitU], (r == 2u));
+      }"
+    `);
+  });
+
+  it('should handle nested runtime ternaries', () => {
+    const myFn = tgpu.fn(
+      [d.u32, d.u32, d.u32, d.u32],
+      d.u32,
+    )((r, v1, v2, v3) => {
+      return r === d.u32(1) ? v1 : r === d.u32(2) ? v2 : v3;
+    });
+
+    expect(tgpu.resolve([myFn])).toMatchInlineSnapshot(`
+      "fn myFn(r: u32, v1: u32, v2: u32, v3: u32) -> u32 {
+        return select(select(v3, v2, (r == 2u)), v1, (r == 1u));
+      }"
+    `);
+  });
+
+  it('should handle bit shift in branch with function param', () => {
+    const myFn = tgpu.fn(
+      [d.bool],
+      d.u32,
+    )((isCustom) => {
+      return isCustom ? d.u32(1) << d.u32(20) : d.u32(0);
+    });
+
+    expect(tgpu.resolve([myFn])).toMatchInlineSnapshot(`
+      "fn myFn(isCustom: bool) -> u32 {
+        return select(0u, (1u << 20u), isCustom);
+      }"
+    `);
+  });
+
+  it('should handle struct field access across ternaries', () => {
+    const Cw = d.struct({
+      low: d.u32,
+      high: d.u32,
+    });
+
+    const myFn = tgpu.fn(
+      [d.bool, Cw, Cw],
+      d.u32,
+    )((isCustom, customCw, stdCw) => {
+      return isCustom ? customCw.low : stdCw.low;
+    });
+
+    expect(tgpu.resolve([myFn])).toMatchInlineSnapshot(`
+      "struct Cw {
+        low: u32,
+        high: u32,
+      }
+
+      fn myFn(isCustom: bool, customCw: Cw, stdCw: Cw) -> u32 {
+        return select(stdCw.low, customCw.low, isCustom);
+      }"
+    `);
+  });
+
+  it('should handle buffer layout access in ternary branches', ({ root }) => {
+    const Cw = d.struct({
+      low: d.u32,
+      high: d.u32,
+    });
+
+    const Layout = d.struct({
+      codewords: d.arrayOf(Cw, 64),
+    });
+
+    const layout = root.createUniform(Layout, d.ref);
+
+    const myFn = tgpu.fn(
+      [d.bool, d.u32],
+      d.u32,
+    )((isCustom, cwIdx) => {
+      return isCustom ? layout.$.codewords[cwIdx]!.low : layout.$.codewords[cwIdx + d.u32(1)]!.low;
+    });
+
+    expect(tgpu.resolve([myFn])).toMatchInlineSnapshot(`
+      "struct Cw {
+        low: u32,
+        high: u32,
+      }
+
+      struct Layout {
+        codewords: array<Cw, 64>,
+      }
+
+      @group(0) @binding(0) var<uniform> layout_1: Layout;
+
+      fn myFn(isCustom: bool, cwIdx: u32) -> u32 {
+        return select(layout_1.codewords[(cwIdx + 1u)].low, layout_1.codewords[cwIdx].low, isCustom);
+      }"
+    `);
+  });
+
+  it('should handle ternary with comparison and unary negation in branches', () => {
+    const myFn = tgpu.fn(
+      [d.u32],
+      d.u32,
+    )((n) => {
+      return n > 0 ? n : -n;
+    });
+
+    expect(tgpu.resolve([myFn])).toMatchInlineSnapshot(`
+      "fn myFn(n: u32) -> u32 {
+        return select(-(n), n, (n > 0u));
+      }"
+    `);
+  });
+
+  it('should throw when a ternary branch contains an assignment', () => {
+    const myFn = tgpu.fn(
+      [d.i32],
+      d.i32,
+    )((a) => {
+      let b = 0;
+      return a > 0 ? (b = a) : 0;
+    });
+
+    expect(() => tgpu.resolve([myFn])).toThrowErrorMatchingInlineSnapshot(`
+      [Error: Resolution of the following tree failed:
+      - <root>
+      - fn:myFn: Ternary operator '(a > 0) ? (b = a) : 0' is invalid. For more complex branching, please use 'std.select' or if/else statements.]
+    `);
+  });
+});